The use of a variety of toughening agents to modify epoxy resins improves the fracture toughness of epoxy systems. Many factors affect the thermal and mechanical properties of such epoxy systems. Epoxy resins are defined as any molecules containing one or more 1,2 epoxy groups, mainly terminated, that can be converted to a thermoset form or a three-dimensional network structure. Depending on the chemical structure of the curing agents and on curing conditions, the properties of cured epoxy resins are versatile, including excellent chemical and heat resistance, high adhesive strength, low shrinkage, good impact resistance, high strength and hardness, and high electrical insulation. But cured epoxy systems have a main drawback: their considerable brittleness, which shows poor fracture toughness, poor resistance to crack propagation and low impact strength. This inherent brittleness has limited their application in fields required high impact and fracture strengths, such as reinforced plastics, matrix for composites and coatings.
Rubber-modified epoxy resins exhibit increased fracture toughness, but also show the deterioration of other important properties compared to the unmodified resin. As was shown in the article [I. Ryb'ev, N. Blank, O. Figovsky. Polymer conglomerates based on epoxy-rubber compositions. Building Materials, 1978, No. 6, pp. 26-27—in Russia] the properties of rubber-modified epoxy resins using as a matrix for anticorrosive composite materials depend not only from types of epoxy resins, liquid rubber and curing agent but also from heterogenic structures of these systems, that could be characterized by an integral indicator—a specific surface of phases division.
Depend of application conditions many patents described different epoxy-rubber composition. Epoxy resin adhesive composition is described in U.S. Pat. No. 4,749,738; it contains an epoxy compound and a reaction product of an aliphatic polyamine with a diene type liquid rubber having a terminal carboxyl group, which is valuable for bonding of FRP. Another composition is disclosed in U.S. Pat. No. 5,075,379 a curing agent for epoxy resins a rubber/diamine blend, more particularly, diaminoisopropylbenzene (DAIPB) modified with an amine terminated butadiene liquid rubber (ATBN). Epoxy adhesive composition is disclosed in Japan patent No. 8218051. This adhesive composition essentially comprises an epoxy resin, modified acrylonitrile-butadiene liquid rubber with terminated carboxyl groups and polyamic acid curing agent. Epoxy resin composition flexibilized with liquid and hydroxyl rubber is disclosed in Chinese patent No. 1277225. This composition is prepared with bisphenol A epoxy resin, organic tinphosphate, liquid rubber having terminated hydroxyl groups.
As a typical example of epoxy-rubber curable crack-resistant composition it is possible to see the composition according U.S. Pat. No. 3,926,903 filled on Sep. 30, 1974 comprises, in weight percent referring to the total weight of components (a) and (b): (a) 92-94% of a bicyclopentadiene ether epoxy resin, (b) 6-8% of a reactive elastomer selected from the group consisting of a functionally terminated butadiene homopolymer, a functionally terminated butadiene-acrylonitrile copolymer and a functionally terminated butadiene-styrene copolymer and c) an aromatic polyamine curing agent in an amount sufficient to provide 0.5-2.0 amino hydrogen atoms of the amine for each epoxy group.